# Release notes

## Overview

The 4.9 release introduces [Serial Groups](https://circleci.com/docs/guides/orchestrate/controlling-serial-execution-across-your-organization/) for job serialization, [Automatic Reruns](https://circleci.com/docs/guides/orchestrate/automatic-reruns/) for workflows and steps, and improved caching with Zstd compression.

## Upgrade

Server 4.9 requires mongoDB 4.4 or higher. If your MongoDB instance is not externalized then please follow our [Upgrade MongoDB to 4.4 guide](https://circleci.com/docs/server-admin/latest/operator/upgrade-mongo/#upgrade-mongodb-to-4) before upgrading to Server 4.9.0.

For upgrade steps see the [Upgrade Server 4.9 Guide](https://circleci.com/docs/server-admin/latest/installation/upgrade-server/).

## Release 4.9.1

For full details of this release see the [changelog](https://circleci.com/changelog/server-release-4-9-1/).

## Release 4.9.0

For full details of this release see the [changelog](https://circleci.com/changelog/server-4-9-0-released/).